Originally Posted by Jakefrom
I believe it depends on the kind of tune you want, you can tune more for performance or economy. 91 burns a little smoother than 87.
Does the ECU in the focus not adjust to octane ratings like other cars? Example, my V6 accord will adjust to different octanes and advance or retard timing to compensate, but switching from one to the other you need to go about 500 miles on the certain octane before the computer recalibrates for the octane. Running 91 instead of 87 will get rid of pinging and get me slightly better milage although on 87 I can average 35mpg on the freeway (about 1 mpg better with 91). A honda engineer has said running 91 instead of 87 will give you a 10hp bump in the accord as well. All this without a tune.
Thanks for bearing with me, trying to expand the knowledge of my vehicles.
No cars can adjust for the octane you are running as there is nothing in the fuel tank that tells the ECU what octane your running so there is no way for a ECU to recalibrate , what they can do is see your o2 signal that may go leaner/rich and adjust with adptive learning and see the knock sensor and take out spark if you have knock